1. Field of the Invention
The present invention relates to a structure of a high-frequency component having a high-frequency element to which a dc bias is applied through a transmission line.
2. Description of the Background Art
As shown in FIG. 6, a conventional high-frequency component such as an antenna switch, for example, comprises a high-frequency element 2 such as a diode or a transistor which is provided on a signal line 1, and first and second transmission lines 3 and 4 which are connected to input and output sides of the high-frequency element 2 respectively. The first and second transmission lines 3 and 4 are formed by striplines or coplanar lines, for example, so that single ends thereof are grounded. Further, capacitors C1 and C2 are provided on the input and output ends of the high-frequency element 2 respectively, while a capacitor C3 is provided between the first transmission line 3 and the ground. In addition, a dc voltage Vc is applied to a node between the first transmission line 3 and the capacitor C3. Therefore, the current at the dc bias Vc is carried to the ground through the first transmission line 3, the high-frequency element 2 and the second transmission line 4. The capacitors C1 to C3 serve as direct current inhibiting capacitors for inhibiting passage of the direct current so that the current of the dc bias Vc flows only in the circuit passing through the high-frequency element 2 and the first and second transmission lines 3 and 4.
The first and second transmission lines 3 and 4 are set at line lengths of .lambda./4 with respect to the wavelength .lambda. of an actuating signal for the high-frequency element 2. In operation of the high-frequency element 2, therefore, the impedances of the first and second transmission lines 3 and 4 are substantially infinitely increased, thereby preventing the signal flowing through the signal line 1 from leaking toward the ground through the transmission lines 3 and 4.
When the high-frequency element 2 has a low operating frequency, i.e., when the actuating signal has a long wave-length, however, the line lengths of the first and second transmission lines 3 and 4 must be increased in response to the wavelength. Thus, the size of the high-frequency element 2 is disadvantageously increased. When the operating frequency of the high-frequency element 2 is varied, further, the line lengths of the transmission lines 3 and 4 must also be varied in response thereto. Thus, it is difficult to cope with various operating frequencies of the high-frequency element 2.